Chemical weapons were first used on a large-scale during the First World War, a conflict that also came to be known as ‘The Chemists’ War’.

German and French troops threw shells at each other containing chlorine and phosgene, causing many who inhaled the gases to die of asphyxiation.

Some 100,000 men lost their lives and a further one million lived with the after-effects.

By 1925, more than 30 countries had signed the Geneva Protocol which banned the use of chemical weapons.

However, the Geneva Protocol did not stop some countries developing their chemical weapons.

German chemists worked on a cyanide-based pesticide called Zyklon B. The Nazis used Zyklon B in the gas chambers to kill an estimated 1.2 million people – mainly Jews, but also Gypsies and gay people. Victims usually died within 20 minutes.

In an effort to deprive guerilla fighters of food, the US army sprayed close to 67 million litres of Agent Orange over Vietnam, eastern Laos and parts of Cambodia during the Vietnam War. It is a herbicide known to lead to adverse effects in humans. Hundreds of thousands of people were burnt or died and even more children were born with birth defects.

Former Iraqi leader Saddam Hussein first used chemical weapons during the war with Iran in the early 1980s.

Later on in the same decade, Iraq’s Kurds became the victims, as Saddam and his Ba’ath party carried out ethnic cleansing in the north of the country. The most infamous attack was on the village of Halabja in March 1988.

Thirteen people were killed and nearly 5,000 injured after apocalyptic group Aum Shinrikyo released sarin nerve gas in the Tokyo subway system in March 1995. Cult leader Shoko Asahara was sentenced to death. The subway attack served as a warning of the possible use of chemicals by terrorists.

There have been allegations of chemical weapons use in Syria since last December, when rebels accused the government of poisoning people in Homs. At the time, many Western countries agreed that if toxic agents were being used, Damascus would be crossing a “red line” that could trigger foreign military intervention.